A printing press comprising such a cylinder body for orienting magnetic flakes is known as such in the art. Such a printing press is for instance disclosed in International application No. WO 2005/000585 filed in the name of the present applicant.
One embodiment of a sheet-fed printing press disclosed in International application No. WO 2005/000585 is represented in FIG. 1. This printing press is adapted to print sheets according to the silk-screen printing process and comprises a feeding station 1 for feeding successive sheets to a silk-screen printing group 2 where silk-screen patterns are applied onto the sheets. In this example the printing group 2 comprises an impression cylinder 2a cooperating with two screen cylinders 2b, 2c placed in succession along the printing path of the sheets. Once processed in the printing group 2, the freshly printed sheets are transported by means of a conveyor system 3 to a delivery station 4 comprising a plurality of delivery pile units, three in this example. The conveyor system 3 is typically an endless chain conveyor system comprising a plurality of spaced-apart gripper bars (not shown in FIG. 1) extending transversely to the sheet transporting direction, each gripper bar comprising clamping means for holding a leading edge of the sheets.
In the example illustrated in FIG. 1, a cylinder 10 carrying a plurality of magnetic-field-generating devices is located along the path of the sheets carried by the chain conveyor system 3. This cylinder 10 is designed to apply a magnetic field to selected locations of the sheets for the purpose of orienting magnetic flakes contained in the patterns of ink or varnish which have been freshly-applied on the sheets in the printing group 2. A drying or curing unit 5 is provided downstream of the cylinder 10 for drying, respectively curing, the ink/varnish applied onto the sheets after the magnetic flakes have been oriented, such unit 5 being typically an infrared drying unit or a UV curing unit depending on the type of ink or varnish used.
Further details regarding silk-screen printing presses, including relevant details of the silk-screen printing press illustrated in FIG. 1, can be found in European patent applications EP 0 723 864, EP 0 769 376 and in International applications WO 97/29912, WO 97/34767, WO 03/093013, WO 2004/096545, WO 2005/095109 and WO 2005/102699, all incorporated by reference to this effect in the present application.
Silk-screen printing is in particular adopted, in the context of the production of security documents, such as banknotes, to print optically-variable patterns onto the documents, including so-called iridescent patterns and OVI® patterns (OVI® is a registered trademark of SICPA Holding SA, Switzerland). Such patterns are printed using inks or varnishes containing special pigments or flakes producing optically variable effects.
So-called “magnetic flakes” are also known in the art, which magnetic flakes have the particularity that they can be oriented or aligned by an appropriately-applied magnetic field. Such magnetic flakes and method for orienting such magnetic flakes are discussed in particular in U.S. Pat. No. 4,838,648, European patent application EP 0 686 675, and International applications WO 02/073250, WO 03/000801, WO 2004/007095, WO 2004/007096, WO 2005/002866, all incorporated by reference to this effect in the present application.
The most convenient method to apply the above magnetic flakes is by silk-screen printing as discussed in the above-mentioned International application WO 2005/000585. This is mainly due to the fact that the flakes have a relatively important size which restricts the choice of available printing processes for applying inks or varnishes containing such flakes. In particular, one has to ensure that the flakes are not destroyed or damaged during the printing process, and silk-screen printing constitutes the most convenient printing process to achieve this goal. Furthermore, silk-screen printing has the advantage that the inks or varnishes used exhibit a relatively low viscosity which favours proper orientation of the magnetic flakes.
Nevertheless, other printing processes could be envisaged to apply inks and varnishes containing magnetic flakes. In European patent application EP 1 650 042, it is for instance proposed to apply such magnetic flakes in an intaglio printing process, whereby the paste-like intaglio ink containing the flakes is heated to decrease the viscosity of the ink and thereby allow the flakes to be oriented more easily. This can be performed in a conventional intaglio printing press, since the plate cylinder of such presses is commonly brought to an operating temperature of approximately 80° C. during printing operations.
Orientation of the magnetic flakes is carried out by applying an adequate magnetic field to the freshly-applied ink or varnish containing the magnetic flakes. By appropriately shaping the field lines of the magnetic field, as for instance discussed in the above-mentioned patent publications, the magnetic flakes can be aligned in any desired pattern producing a corresponding optically-variable effect which is very difficult, if not impossible to counterfeit.
As already mentioned hereinabove, an adequate solution for orienting the magnetic flakes consists in bringing the sheets in contact with a rotating cylinder carrying a plurality of magnetic-field-generating devices.
Referring again to FIG. 1, and as discussed in International application No. WO 2005/000585, the cylinder 10 could alternatively be located at the sheet transfer location 3a between the impression cylinder 2a and the conveyor system 3. Still according to another embodiment envisaged in International application No. WO 2005/000585, the impression cylinder 2a itself could be designed as a cylinder carrying magnetic-field-generating devices.
In the embodiment illustrated in FIG. 1, the cylinder 10 used to orient the magnetic flakes advantageously cooperates with the non-freshly-printed side of the sheets, thereby preventing smearing problems, the magnetic field being applied from the back side of the sheets through the freshly-printed patterns of ink or varnish. During orientation of the magnetic flakes, i.e. at the time when a sheet carried by the conveyor system 3 contacts the upper part of the circumference of the cylinder 10, the cylinder 10 is rotated at a circumferential speed corresponding to the speed of the transported sheets so that there is no relative displacement between the transported sheets and the circumference of the cylinder. As illustrated, the cylinder 10 is placed in the path of the chain conveyor system 3 such that the sheets follow a curved path tangential to the outer circumference of the cylinder 10, thereby enabling part of the surface of the processed sheet to be brought in contact with the outer circumference of the cylinder 10.
In the context of the production of banknotes, in particular, each printed sheet (or each successive portion of a continuous web, in case of web-printing) carries an array of imprints arranged in a matrix of rows and columns, which imprints ultimately form individual securities after final cutting of the sheets or web portions. The cylinder used to orient the magnetic flakes is therefore typically provided with as many magnetic-field-generating devices as there are imprints on the sheets or web portions.
The format and/or layout of the printed sheets (or successive web portions) depends on each case, in particular on the dimensions of each individual imprint and the number thereof. This means that the magnetic cylinder must be configured accordingly.
There is therefore a need for an adaptable cylinder configuration which enables quick adaptation thereof to a new format and/or layout of the printed substrate.